Abstract Perinatal arterial ischemic stroke is common and produces significant morbidity and severe long-term neurological deficits. Perinatal stroke leads to significant morbidity and severe long-term neurological and cognitive deficits. More than half of all children with cerebral palsy are born at term. It has become clear that the maturational stage of the brain at the time of injury plays a key role in the pattern of brain damage in humans but the underlying mechanisms of the differing pathophysiology of perinatal stroke and stroke in the adult are still poorly understood. The choroid plexuses (CPs) may protect the brain as physical and biochemical barriers and by modifying neuroinflammation. Until recently CPs were neglected as a potential target for therapy but are now considered as pharmacological target for treatment of neurodegenerative diseases. We will investigate the neuroprotective role of the CPs against neonatal stroke induced in an age-appropriate stroke model, a transient middle cerebral artery occlusion (tMCAO) in postnatal day 9 (P9) mice. In particular, we will determine how subsets of monocytes that enter through the CPs protect and whether toll-like receptor 2 (TLR2), one of key components of the innate immune system, plays a central role in CPs-mediated neuroprotection. We hypothesize that monocyte trafficking through CPs protects neonatal brain against tMCAO in a TLR2 dependent manner. We will determine relationships between the monocyte phenotypes entering through CPs and inflammation and injury after tMCAO in neonatal mice (Aim1) and define the mechanisms of TLR2- mediated monocyte trafficking via functional or dysfunctional CPs on injury (Aim 2). We will use unique Cx3Cr1GFP/+/CCR2RF/P+ mice to distinguish monocytes from microglia and characterize monocyte/microglial phenotypes and use double-reporter luc/GFP-TLR2 mice to non-invasively monitor the spatial-temporal pattern of TLR2 upregulation in real time in living injured mice. Our ability to identify injured neonatal mice during tMCAO by diffusion-weighted MRI (DWI) combined with various biochemical assays, such as multi-color flow cytometry, EMSA and cytokine multiplex, in mice with inhibited function of CPs or/and TLR2 will enhance the understanding of the role of CPs as protective barrier in neonatal stroke and provide new insights on how to alleviate injury after acute neonate stroke and identify pharmacological targets at the blood-CSF barrier.